To be fair, it's kind of a self-perpetuating problem. In America at least, "blond" and "blonde" are used basically interchangeably and no one uses "brunet." So no one in fandom uses -et for male characters because they... don't know it's even a thing. I can't really blame them for that.
Okay, something about the hair color thing that's been bugging me for a while: I think the original example of it was a joke, and one that was actually mildly witty at that. Specifically, 'bluenette'. Like, given how it sounds I'm about sixty percent sure it was supposed to be a pun on 'brunette'. And the first time I saw it, I chuckled a little because, yes, anime characters have blue hair the same way people in real life have brown hair, that's a funny little joke, good for them. Aaaand then I branched out into the wider parts of anime fandom, and realized that if it had been originally a joke, then a bunch of people really hadn't gotten it and now they all just thought 'ette' was what you slapped onto the end of any color to make it about hair. It could have been funny, dammit.
Well, gray/white hair is caused by the absence of pigment... So maybe it's describing someone whose pigment-producing cells have given up the ghost and whose hair is technically clear, e.g. free of pigments.
I will admit, I have used 'silverette', but the character in question using the term was completely gooey-eyed and infatuated with the grey-haired character at the time, and it was really meant as "He's so in love he's developed CHEESE CANNONS". But even then, it's a bad habit, and I have since stopped using it at all.
Kind of related: as I get older, I keep progressively toning down the crazy hair colors I gave characters I created back when I was, like, 14. "It's BRIGHT GREEN!" -> "It's a grass green." -> "Nah, it's more a muted olive." -> "His hair has a slight olive overtone." -> "... it's brown."
I used "silver-blonde" for silver hair and "strawberry blonde" for pink. I made the excuse that at least I didn't use epithets, so these terms only came up when characters were actually talking about hair for some reason. (Sweet Athe, "brownette"? Someone, somewhere has failed at words.)
it bugs me when the author has clearly designated one or two characters to be Knowledgeable about, say, gender things or queerness or asexuality or something and its like... im sorry if you want someone to help your main character understand their orientation/gender that theyre just sort of realizing, maybe dont just slap a personality onto someone and have them function solely as Gentle Supportive Cishet (alternately sometimes theyre queer too) like "yes of course bruce banner would A) know all this shit; and B) feel like explaining it immediately and with no awkwardness" i dont have any specific examples of this but i did encounter it when i read more MCU fics
How about people using the semi-academic Tumblr-approved terms for sexuality things in fic set in non-modern settings? e.g. Legolas is not going to know the word "demisexual."
(I had problems with Clariel, but I appreciate how much thought the author put into coming up with a term to unambiguously convey "aro ace" that didn't sound anachronistic and jar the reader out of the story. Please learn from this, other writers.)
Related is when the one supportive person in a uniformly homophobic setting is conveniently also queer. Because nobody could possibly be conflicted about what their friend's doing and love and support them anyway because that might be interesting. Bonus if, say, the main pair are gay dudes and their cheerleader's a lesbian so there's no potential competition and therefore nothing interesting happening there either.
And I should also add that it's kind of weird when the queer character doesn't appear to have grown up with these views everyone's suddenly expressing, yet isn't surprised by them being expressed. There's no conflict or concern that these views they've supposedly heard all their life might be right, nope, they're just the one magically enlightened person in the setting. Happens with other issues too, but queerosity is the topic right now.
I've seen a lot of that pattern with the aforementioned Girl Proving Sexism Wrong stories, too. They just kinda spring into existence with the author's own opinions fully-formed out of the aether.
My friend who made the Quirrell comment has a second fanfic nemesis, named The Last War. His primary gripe with that one is that, I paraphrase, for all its attempts to demonise the Weasleys, Harry and Hermione come out looking even worse. Ron is a caricature of a domestic abuser who Hermione only married to stay close to Harry, unsuccessfully since they haven't spoken in years even though Ron and Harry still work together, and she ends up brutally murdering him in front of their children; Ginny is a fat adulteress and Harry hits her and throws her out of her own home apparently solely over the former. Also been reading a review of the Sword of Truth series, which said "To be perfectly frank, if the villains weren't rapists, we'd have no way of knowing who the bad guy's supposed to be." Then we have my fanfic nemesis where the supposed heroes' response to overhearing a guy be tortured to death was to start fucking.